1. Field of the Invention
The present invention relates to sonars using ultrasonic waves such as a fish finer and an echo sounder that are mounted on a ship.
2. Description of the Related Art
A sonar is an ultrasonic apparatus that emits an ultrasonic wave from a transducer attached to a hull into the water, receives echoes of the ultrasonic wave reflected on a shoal of fish, a water bottom, and the like and returned to the sonar, and displays underwater information. For example, in the case of a fish finder, echoes from the water are distinguished by colors according to signal intensities and displayed on a display unit. As an example, strong echoes returned from a shoal of fish and a water bottom with large reflection intensities are displayed in reddish brown and a weak echo returned from a shoal of fish with a small reflection intensity is displayed in blue, green, or the like.
In such a sonar, since one transducer performs both operations of transmission and reception, an unnecessary signal like ringing appears immediately after an ultrasonic wave is transmitted. This signal is hereinafter referred to as a “tailing signal”. FIGS. 7A and 7B are diagrams for explaining a tailing signal. FIG. 7A is a diagram of a signal waveform of an ultrasonic wave transmitted from the transducer. A tailing signal indicted by a broken line follows a transmission signal indicated by a solid line. The tailing signal is caused by reflection of the transmission signal in the transducer, a band-limited frequency characteristic of an element, and the like. FIG. 7B is a diagram of a signal waveform received by the transducer. Transmission and reception in the transducer are switched by a transmission/reception switching unit. However, since electric power of the transmission signal is extremely large, the transmission signal leaks to a reception circuit from the transmission/reception switching unit. Consequently, as indicated by an alternate long and short dash line in FIG. 7B, a signal of the same waveform as the transmission signal appears. A broken line indicates a tailing signal immediately after transmission of an ultrasonic wave and a solid line indicates an echo signal reflected on a target in the water.
When the tailing signal described above is generated, as shown in FIG. 8, a video 24 of the tailing signal appears on a screen 8a of the display unit 8. Reference signs 20a and 20b denote fish shoal videos. Reference numeral 21 denotes a water bottom video; 22, a water depth scale; and 23, an oscillation line indicating a transmission position of an ultrasonic wave. The video 24 of the tailing signal is a stripe-like color video consisting of three layers of, for example, reddish brown, yellow, and blue in order from above. Consequently, a video 20b of a surface layer fish shoal present near the water surface is covered with the video 24 of the tailing signal to be less easily seen. In some cases, the video 20b of the surface layer fish shoal is completely hidden and cannot be visually recognized.
As measures against this problem, it is conceivable to reduce a time width of the tailing signal and control a signal level by providing a backing material for absorbing sound waves in the transducer. However, when this method is used, the number of components and processing man-hours increase to cause an increase in cost and it is impossible to expect a significant improvement effect. On the other hand, if reception sensitivity is lowered immediately after transmission of an ultrasonic wave, the video 24 of the tailing signal is not displayed. However, the fish shoal videos 20a and 20b, which should originally be displayed, are not displayed either. Thus, this method is unrealistic.
Note that many technologies for controlling reverberation in the water immediately after transmission of an ultrasonic wave in an ultrasonic apparatus have been proposed as described in, for example, JP-A-2001-108738, JP-A-4-116488, and JP-A-2-278180. However, all of the techniques are techniques for controlling reverberant signals generated in the water and are not techniques for controlling a tailing signal caused by a transmission signal in the transducer. Specific technical contents described in the patent documents are different from a technique of the invention described later.